The invention relates to a method for operating an internal combustion engine of a vehicle, in particular a motor vehicle.
In current automotive engineering, spark ignition engines as internal combustion engines with direct gasoline injection are preferred over conventional manifold injection, since these internal combustion engines compared to conventional spark ignition engines have distinctly more dynamics, are superior with respect to torque and output, and at the same time enable a reduction in fuel consumption by up to 15%. This is made possible by so-called stratification in the partial load range, in which an ignitable mixture is required only in the area of the spark plug, while the remaining combustion chamber is filled with air. Since conventional internal combustion engines which operate according to the manifold principle can no longer be ignited at such a high air excess as is present in direct gasoline injection, in this stratification mode the fuel mixture is concentrated around the spark plug which is positioned centrally in the combustion chamber, while pure air is present in the edge areas of the combustion chamber. In order to be able to center the fuel mixture around the central spark plug positioned in the combustion chamber, a dedicated air flow in the combustion chamber is necessary, a so-called tumble flow. An intensive, roller-shaped flow is formed for that purpose and fuel is injected only in the last third of the upward motion of the piston. The combination of the special air flow and the dedicated geometry of the piston which has for example a pronounced fuel flow depression, concentrates the especially finely atomized fuel in a so-called “mixture ball” optimally around the spark plug and reliably ignites it. The engine control or engine control device provides for the respectively optimum adjustment of injection parameters (injection instant, fuel pressure).
These internal combustion engines can therefore be operated for a correspondingly long time in lean operation; this overall has a beneficial effect on fuel consumption, as has already been described in the foregoing. This lean operation however entails the disadvantage of a much greater amount of nitrogen oxide in the exhaust gas so that the nitrogen oxides (NOx) in the lean exhaust gas can no longer be completely reduced with a three-way catalyst. In order to keep nitrogen oxide emissions within prescribed limits, for example the Euro-IV boundary value, nitrogen oxide storage catalysts are used additionally in conjunction with such internal combustion engines. These nitrogen oxide storage catalysts are operated such that in them the large amounts of nitrogen oxides which are produced by the internal combustion engine are stored. As the amount of stored nitrogen oxides increases, a saturation state in the nitrogen oxide storage catalyst is reached so that the nitrogen oxide storage catalyst must be discharged. To this end, for a so-called discharge phase, switching takes place briefly to substoichiometric, rich engine operation by means of the engine control or engine control device, in which the internal combustion engine is operated with a rich mixture which has a shortage of air. At the start of this discharge phase, the oxygen reservoir of the nitrogen oxide storage catalyst is generally emptied, by which the oxygen which is necessary for the discharge process is made available. In this discharge process the stored nitrogen oxide is reduced to nitrogen (N2) especially by the hydrocarbons (HC) and carbon monoxide (CO) which are present in a large amounts under these rich operating conditions; this nitrogen can then be released into the environment. Operating the internal combustion engine of a motor vehicle in a first operating range as the lean operating range is already known in general; in this first operating range the internal combustion engine is operated with a lean mixture which has an air excess and thus an oxygen excess, and the nitrogen oxides produced by the internal combustion engine are stored in a nitrogen oxide storage catalyst, to discharge the nitrogen oxide storage catalyst by means of an engine control device switching from the lean operating range to the rich operating range taking place, in which the internal combustion engine is operated with a rich mixture which has a shortage of air and in which the nitrogen oxides stored in the nitrogen oxide storage catalyst during the lean operating range are discharged from the nitrogen oxide storage catalyst. Furthermore there is a second operating range as a homogeneous operating range in which the internal combustion engine is operated with an essentially stoichiometric homogeneous mixture (lambda=1), switching between the lean operating range and the homogeneous operating range being undertaken by the engine control device depending on the operation-dictated load requirement and/or rpm requirement when a definable switching condition is reached, and switching taking place by the engine control device into the rich operating range first for discharge of the nitrogen oxide storage catalyst before switching from the lean operating range to the homogeneous operating range. Specifically, the lean operating range in this instance is for example a stratified one in conjunction with a dynamic driving style, as is the case for example in city driving, switching generally takes place by the engine control device based on the lean operating range in which the lambda value is approximately 1.4, in particular based on the operation-dictated increased load requirement and/or rpm requirement, into the homogenous operating range, in which the internal combustion engine is operated essentially with a stoichiometric homogeneous mixture of lambda=1. Before switching into the homogeneous operating range, the engine control device switches first into the rich operating range in order to discharge the nitrogen oxide storage catalyst. Research has shown that in this operating mode, in spite of temporary lean operation, the theoretical lean operation fuel savings potential which is actually present is not fully exhausted. Another problem here is that in a very dynamic driving style it is necessary to depart from the lean operating range due to an increased demand for torque more often under certain circumstances, by which then each time there is a need for nitrogen oxide storage catalyst discharge, i.e., a rich operating phase. This also leads to increased fuel consumption.
Similar process guidance is known from the generic DE 100 64 279 A1, in which, depending on the deterioration of the exhaust gas composition, switching takes place between lean, rich and homogeneous operation. The switching decision is made depending on the deterioration of the storage capacity of the nitrogen oxide storage catalyst which is designated as the NOx absorption means. In particular, when a deterioration of the efficiency of the nitrogen oxide storage catalyst is ascertained, lean operation which is designated as an oxygen excess-air-fuel ratio operation is to be blocked.
DE 197 53 718 C1 discloses a process for operating a diesel engine which comprises an engine control which controls operation of the diesel engine depending on the engine characteristics and which enables rich/lean control of the diesel engine. The engine control comprises a computer which effects switching to rich or lean operation of the diesel engine depending on predetermined switching criteria. Furthermore, there are sensors which communicate with the computer and which monitor the parameters are necessary for the switching criteria, and a memory which communicates with the computer, in which the engine characteristics are stored for operation of the diesel engine. The computer effects switching from lean to rich operation when the maintenance of a regeneration temperature of the storage catalyst element through which the exhaust gases of the diesel engine have flowed and the presence of a predetermined charging state of the storage catalyst element through which the exhaust gases of the diesel engine have flowed are satisfied as the switching criteria. Furthermore, the computer effects switching back from rich to lean operation when one of the switching criteria for switching from lean to rich operation is not present or a regeneration time has elapsed which depends on the respective charging state of the storage catalyst element through which the exhaust gases of the diesel engine have flowed at the start of the rich operating phase, or there is a predetermined content of the reducing agent in the exhaust gases downstream from the storage catalyst element or the exhaust gas temperature is below a predetermined threshold value.
Furthermore, in the dissertation of Andreas Hertzberg (Stuttgart 2001) entitled “Operating strategies for a spark ignition engine with direct injection and a NOx storage catalyst” in Chapter 6, especially under item 6.4.2, tests on operation of an internal combustion engine in lean operation are described and evaluated. Here the focus was especially on the consumption difference of various test driving cycles depending on torque threshold values.